Bridges are structures built to span a valley, road, railroad track, river, body of water or any other physical obstacule. Desings of bridges will vary depending of the function of the bridge.

Which bridge type is eventually selected depends on many factors including the length to be spanned, the loads to be carried, and the cost to make and erect the bridge. The conditions below the area being crossed also play a large role in the kind of bridge selected.

The first bridges were made by nature. The first bridges made by humans were probably spans of wooden logs or planks and eventually stones, using a simple support and crossbeam arrangement. Most of these early bridges could not support heavy weights or withstand strong currents. It was these inadequacies which led to the development of better bridges.

The Beam bridge

The design is as simple as a single rigid beam, resting on supports at either end and unsupported in the middle. The weight of the beam, and of any traffic on it, is carried directly to the ground by the supports, often called 'piers' in the trade.

The beam need not be of any particular shape and there are no other elements besides the piers to help dissipate the load. Hence the piers take the full weight of the load and are said to be in compression. This means that they are being squashed by the forces at the top and bottom, and must be built from materials that can resist such forces without crumpling.

Types of Beam bridges

The Arch bridge

An arch bridge is a semicircular structure with abutments on each end. The design of the arch, the semicircle, naturally diverts the weight from the bridge deck to the abutments. Arch bridges are always under compression. The force of compression is pushed outward along the curve of the arch toward the abutments.

The tension in an arch is negligible. The natural curve of the arch and its ability to dissipate the force outward greatly reduces the effects of tension on the underside of the arch.

The Suspension Bridge

A suspension bridge is one where cables are strung across the obstacle and the deck is suspended from these cables. Modern suspension bridges have two tall towers through which the cables are strung. Thus, the towers are supporting the majority of the roadway's weight.

The force of compression pushes down on the suspension bridge's deck, but because it is a suspended roadway, the cables transfer the compression to the towers, which dissipate the compression directly into the earth where they are firmly entrenched.

The supporting cables, running between the two anchorages, are the lucky recipients of the tension forces. The cables are literally stretched from the weight of the bridge and its traffic as they run from anchorage to anchorage. The anchorages are also under tension, but since they, like the towers, are held firmly to the earth, the tension they experience is dissipated.

Types of Suspension Bridges

Suspension bridges come in two different designs: the suspension bridge, recognized by the elongated 'M' shape, and the less-common cable-stayed design, which has more of an 'A' shape. The cable-stayed bridge does not require two towers and four anchorages as does the suspension bridge. Instead, the cables are run from the roadway up to a single tower where they are secured. The tower in a cable-stayed bridge, like its counterpart in a suspension bridge, is responsible for absorbing and dealing with the compression forces. In both bridges, the cables are under tension.

Bridges are structures built to connect to points separated by an obstacle. There are six main types of bridges: beam bridges, arch bridges, suspension bridges, cable-stayed bridges, truss bridges and cantilever bridges.

To built a bridge, many years ago, people used wood. Later, bridges were made with stones and currently engineers use steel, concrete, steel wires and other modern materials.

An important advantage of built a bridge is that connet to points that were separeted and allows people go to one side to the other. Also increases the trade between those to points, that could be two diferents countries.

How bridges work

Bridges-Wikipedia